Oxidation-resistant and wear-resistant polyethylenes for human joint replacements and methods for making them

ABSTRACT

The present invention presents methods for making oxidation-resistant and wear-resistant polyethylenes and medical implants made therefrom. Preferably, the implants are components of prosthetic joints, e.g., a bearing component of an artificial hip or knee joint. The resulting oxidation-resistant and wear-resistant polyethylenes and implants are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 10/258,762, filed Oct. 25, 2002, entitled“Oxidation-Resistant and Wear-Resistant Polyethylenes for Human JointReplacements and Methods for Making Them,” attorney docket number040398-0095, which is a national phase filing of Patent CooperationTreaty Application No. PCT/US01/13839, filed on Apr. 27, 2001, entitled“Oxidation-Resistant and Wear-Resistant Polyethylenes for Human JointReplacements and Methods for Making Them,” which in turn is acontinuation-in-part of U.S. provisional patent application Ser. No.60/200,525, filed on Apr. 27, 2000, entitled “Oxidation-Resistant andWear-Resistant Polyethylene for Human Joint Replacements.” Thisapplication is also related to U.S. patent application Ser. No.11/805,867, filed May 24, 2007 (now U.S. Pat. No. 8,658,710, issued Feb.25, 2004), entitled “Oxidation-Resistant and Wear-ResistantPolyethylenes for Human Joint Replacements and Methods for Making Them,”attorney docket number 040398-0096. The entire content of each of theseapplications is expressly incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to polyethylenes. It discloses methods forenhancing the wear resistance of polyethylenes without inducingoxidative degradation, by crosslinking oxidation-resistantpolyethylenes. The polyethylenes disclosed herein are useful for makingarticles such as medical implants, for example, components of artificialjoints such as acetabular cups.

BACKGROUND OF THE INVENTION

Ultrahigh molecular weight polyethylene (hereinafter referred to as“UHMWPE”) is commonly used to make prosthetic joints such as artificialhip joints. A conventional way of making implants is to machine them outof extruded UHMWPE or block molded UHMWPE. Alternatively, the implantsare directly molded from UHMWPE. The implants are then packaged and nextsterilized with radiation, gas plasma, or ethylene oxide. The sterilizedpackaged implants are then sold and the implants are removed from theirpackages when the surgeons are ready to place them in the patients'bodies.

In recent years, it has become increasingly apparent that wear ofacetabular cups of UHMWPE in artificial hip joints introduces manymicroscopic wear particles into the surrounding tissues. The reaction tothese particles includes inflammation and deterioration of the tissues,particularly the bone to which the prosthesis is anchored. Eventually,the prosthesis becomes painfully loose and must be replaced.

Improving the wear resistance of the UHMWPE socket and, thereby,reducing the rate of production of wear debris would extend the usefullife of artificial joints and permit them to be used successfully inyounger patients. Consequently, numerous modifications in physicalproperties of UHMWPE have been proposed to improve its wear resistance.

UHMWPE components are also known to undergo a spontaneous,post-fabrication increase in crystallinity and changes in other physicalproperties. {see e.g., Rimnac, C. M., et al., J. Bone & Joint Surgery,76-A(7):1052-1056 (1994)}. These changes occur even in stored(non-implanted) prostheses after sterilization with gamma radiation(“gamma radiation sterilization”) which is typically used forsterilization irradiation. The industrial standard for the gammasterilization dose is between 2.5 Mrad to 4 Mrad. Typically, 3 to 3.5Mrad is used.

Gamma radiation initiates an ongoing process of chain scission,crosslinking, and oxidation or peroxidation involving the free radicalsformed by the irradiation. These degradative changes may be acceleratedby oxidative attack from the joint fluid bathing the implant. Oxidationdue to post-irradiation aging of UHMWPE has been associated withpitting, delamination and fracture in total hip replacements. (Walsh,H., et al., “Factors that Determine the Oxidation Resistance of Molded1900: Is it the Resin or the Molding”, Poster Session—Polyethylene, 46thAnnual Meeting, Orthopaedic Res. Soc., Mar. 12-15, 2000, Orlando, Fla.,U.S.A., page 543, first sentence. Hereinafter referred to as “Walsh p.543, supra”). Additionally, many studies have shown that oxidation dueto open-air gamma-irradiation sterilization of UHMWPE components isassociated with delamination and fracture in artificial knee-joints.{Mori, A., et al., “Mechanical Behavior of UHMWPE When Mixed withVitamin E”, Nakashima Medical Division, Nakashima Propeller Co., Ltd.,hand-out at the 47th Ann. Mtg., Orthopaedic Res. Soc., Feb. 25-28, 2001,San Francisco, Calif., first sentence. Hereinafter referred to as “MoriHand-Out”.} In another publication, Mori et al. indicate that“Apparently, it is widely recognized through numerous research projectsthat the largest cause of wear on [UHMWPE], used as the bearing surfacein artificial joints, is the oxidative degradation following γ-rayirradiated sterilization in air.” (Mori et al., “Effects ofManufacturing Method and Condition on UHMWPE Wear”, Society forBiomaterials, Sixth World Biomaterials Congress Transactions, p 1122,year 2000, first sentence. Hereinafter referred to as “Mori p. 1122,supra”).

Consequently, several companies have modified the method of gammaradiation sterilization to improve the oxidation-resistance and thus thewear resistance of the UHMWPE components. For example, in the case ofUHMWPE acetabular cups, this has typically involved packaging the cupseither in an inert gas (e.g., Stryker-Osteonics-Howmedica, Inc.,Rutherford, N.J., USA), in a partial vacuum (e.g., Johnson & Johnson,Inc., New Brunswick, N.J., USA) or with an oxygen scavenger (e.g.,Sulzer, Inc., Winterthur, Switzerland) during gamma radiationsterilization. Alternatively, gas plasma or ethylene oxide are used tosterilize the implant in order to avoid the free radicals generated byirradiation sterilization.

Further, in recent years, there have been reports that some UHMWPE maybe particularly resistant to oxidation. Starting from the 1970's, ZimmerInc. (Warsaw, Ind., USA) has been making an UHMWPE commonly known in theindustry as “net-shaped molded UHMWPE” (also known as “direct-moldedUHMWPE”).

Walsh, H., et al., reported that “components made by directly molding1900 resin are resistant to post irradiation aging for periods up to 10years. Further, these directly molded devices have excellent clinicaltrack records and exhibit extremely low incidences of pitting,delamination and fracture”, and that “components made by direct moldingof 1900 resin have a unique resistance to oxidation. Machined componentsfrom extruded 1900 bar or compression molded 1900 sheet do not exhibitthe same oxidation resistance. It is also clear from Table 1 thatextruded bar and compression molded sheet of 4150 resin are alsosusceptible to post irradiation aging . . . . Table 2 shows thatdirectly molding either 4150 or 1990 resin provides anoxidation-resistant material. As the resin used in the directly molded4150 was the same lot as that used in the extruded bars, the onlydifference between the samples was the manufacturing method.” (Leftcol., first and second sentences, and right col., second and thirdparagraphs of Walsh p. 543, supra).

Since the rate of oxidation is slow, and typically takes 5 years ofpost-irradiation aging (i.e., 5 years of shelf aging) for density valuesto rise above 0.95 g/cc, an indicator of oxidation, Walsh et al reportedan accelerated aging protocol to mimic 5 years shelf aging of UHMWPE.(Walsh, H. A., et al., “A True, Reproducible Accelerated Aging ProtocolTo Mimic 5 Year Shelf Aging of UHMWPE”, Poster Session—Polyethylene,46th Ann. Mtg., Orthopaedic Res. Soc., Mar. 12-15, 2000, Orlando, Fla.,U.S.A., page 542. Hereinafter referred to as “Walsh p. 542, supra”).

Walsh p. 542, supra, reported that specimens directly molded from 4150HPpowder to provide a modulus of 600-800 MPa did not oxidize, when agedwith its accelerated aging protocols, as compared to HSS/PolyHiSolidur/Ticona reference extruded 4150HP, which was machined intospecimens.

On the other hand, Mori p. 1122, supra, compared specimens: (1) machinedfrom GUR 4150HP ram extruded bar stock; (2) custom molded directly fromGUR1150 resin and using compression pressure of 45 kgf/cm²; and (3)custom molded directly from GUR1150 resin and using compression pressureof 200 kgf/cm². Mori et al., observed that directly compression moldedspecimens with 45 kgf/cm² compression pressure and ram extrudedspecimens showed highly-developed oxidation compared to the 200 kgf/cm²directly compression molded specimen. Thus, Mori et al., reported that:“It is generally said that direct compression molded UHMWPE provide anunexpected resistance to oxidation. However, the results obtained fromthis study reveal that granted directly compression molding method wasused, unsuitable conditions of the process could cause less resistanceto oxidation.” (Mori p. 1122, supra, first paragraph under the“Discussion” section).

In another paper, Mori, A., et al., compared two groups of UHMWPEspecimens made from: (1) directly molded from UHMWPE powder (GUR1020,Ticona) at 10 MPa, and (2) directly molded from a mixture of GUR1020powder and Vitamin E, also at 10 MPa. Mori et al., then gamma sterilizedin air (at 25 kGy which is equivalent to 2.5 Mrad) and then appliedaccelerated aging techniques to both groups. Mori et al reported thatthe addition of Vitamin E to UHMWPE powder provided a unique resistanceto oxidation, and preserved the initial mechanical properties. (Mori etal., “Mechanical Behavior of UHMWPE When Mixed with Vitamin E”, PosterSession—Polyethylene, 47th Ann. Mtg., Orthopaedic Res. Soc., Feb. 25-28,2001, San Francisco, Calif., p. 1017. Hereinafter referred to as “Morip. 1017, supra”). N. Tomita, a co-author of Mori p. 1017, supra, haspreviously reported that his gamma-irradiated vitamin-E-containingspecimens demonstrated no subsurface crack formation and no flaking-likedestruction. {Tomita, N., et al., “Prevention of Fatigue Cracks inUltrahigh Molecular Weight Polyethylene Joint Components by the Additionof Vitamin E”, J. Biomed Mater Res (Appl Biomater) 48: 474-478 (1999)}.

Mori et al teach that: “Recently the practice of crosslinking UHMWPEafter gamma-ray or electron-beam irradiation has developed to improveresistance to oxidation. However, the process of crosslinking of UHMWPEis complex, and it is difficult to remove the free radicals created byirradiation. It is suggested that the addition of harmless Vitamin E isa very simple and extremely effective method to prevent oxidationdegradation and maintain the stability of UHMWPE component in artificialjoints.” (Mori 1017, supra, paragraph bridging left and right columns.See also, Mori hand-out, second and third last sentences).

SUMMARY OF THE INVENTION

The present invention presents methods for making and selecting foroxidation-resistant and wear-resistant polyethylenes and medicalimplants made therefrom. Preferably, the implants are components ofprosthetic joints, e.g., a bearing component of an artificial hip, knee,or other joint. The resulting oxidation-resistant and wear-resistantpolyethylenes and implants are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically shows the oxidation profiles of slow-cooled andquenched crystallized UHMWPE that were gamma radiation sterilized in airat an average dose of 3.4 Mrad and subsequently stored in air at roomtemperature for about 6.5 years.

DETAILED DESCRIPTION OF THE INVENTION

The invention described herein draws on previously published work andpatent applications. By way of example, such work consists of scientificpapers, abstracts, posters, hand-outs, and patents. All of thesepublications and patent applications, cited previously or below, arehereby incorporated by reference to the same extent as if eachindependent publication or patent application was specifically andindividually indicated to be incorporated by reference.

I. Choices of Polyethylenes

Abbreviations used in this application are as follows:

-   -   PE polyethylene    -   UHMW ultra-high molecular weight    -   UHMWPE ultra-high molecular weight polyethylene HMW high        molecular weight    -   HMWPE high molecular weight polyethylene

The present invention relates to polyethylenes in general, andpreferably HMWPE and UHMWPE. For ease of discussion, the followingdiscussion uses HMWPE and UHMWPE as examples of a polyethylene and isnot intended to limit the invention to merely HMWPE and UHMWPE. Thisinvention also presents finished articles (i.e., end products) made ofpolyethylenes. Non-limiting examples of these finished articles aremedical implants. Again, for ease of discussion, the followingdiscussion uses an acetabular cup as a non-limiting example of a medicalimplant and is not intended to limit the invention to merely anacetabular cup.

The preferred polyethylenes are HMW polyethylene (HMWPE) and UHMWpolyethylene (UHMWPE). HMWPE have molecular weights ranging from about10⁵ grams per mole to just below 10⁶. UHMWPE have molecular weightsequal to or higher than 10⁶ grams per mole, preferably from 10⁶ to about10⁷. The polyethylenes are generally between about 400,000 grams permole to about 10,000,000.

For implants, the preferred polyethylenes are those that are wearresistant and have exceptional resistance to chemical degradation.UHMWPE is the most preferred polyethylene as it is known for theseproperties and is currently widely used to make acetabular cups fortotal hip prostheses and components of other joint replacements.Examples of UHMWPE are those having molecular weight ranging from about1 to 8×10⁶ grams per mole, examples of which are: GUR 4150 or 4050(Hoechst-Celanese Corporation, now known as Ticona, League City, Tex.)with a weight average molecular weight of 5 to 6×10⁶ grams per mole; GUR4130 with a weight average molecular weight of 3 to 4×10⁶; GUR 4120 or4020 with a weight average molecular weight of 3 to 4×10⁶; RCH 1000(Hoechst-Celanese Corp.) with a weight average of molecular weight of4×10⁶ and HiFax 1900 of 2 to 4×10⁶ (HiMont, Elkton, Md.).

Ticona is the world's largest producer of medical grade UHMWPE.Historically, companies which make implants have used polyethylenes suchas HIFAX 1900, GUR 4020, GUR 4120 and GUR 4150 for making acetabularcups.

UHMWPE is commercially available in two forms, one with and the otherwithout calcium stearate. Examples of UHMWPE with calcium stearate areGUR 4120, 4130, 4150 and RCH 1000. Examples of UHMWPE without calciumstearate are GUR 1050 (also known as GUR 4050), 1020 (also known as GUR4020), and HiFax 1900. Both forms of UHMWPE are within the definition of“UHMWPE” as used in this patent application. For the nomenclature anddiscussion of the different UHMWPE, their nomenclature andmanufacturers, See Kurtz, S. M., et al., “Advances in the processing,sterilization, and crosslinking of ultra-high molecular weightpolyethylene for total joint arthroplasty”, Biomaterials 20: 1659-1688(1999)(hereinafter referred to as “Kurtz, et al., supra”).

Polyethylenes can generally be obtained in any ground, morselized, orpelleted form that can be molded or extruded into a larger solid piece.In this form, the polyethylenes are usually available as “polyethylenepowder”, “polyethylene flake” and “polyethylene resin” which aresynonymous terms. For ease of discussion, the following discussion willuse the term “Polyethylene powder”. Thus, one skilled in the art wouldrealize that, if he wishes, he could replace the “polyethylene powder”discussed in this application (including its claims) with any ground,morselized, or pelleted form of polyethylene.

“Net shaped molding” and “direct molding” are synonymous terms used inthe art; so are “net-shaped molded” and “direct-molded”. For ease ofdiscussion, this application will use the term “direct molding” and“direct-molded”. Some commercially available polyethylene implants areproduced by direct molding; that is, by placing the originalpolyethylene powder into a mold and applying sufficient heat andpressure to fuse the powder into the final shape of the implant oralmost the final shape of the implant such that only minimalfinish-machining is needed to make the implant.

In contrast, instead of direct molding, the majority of UHMWPEcomponents used in artificial joints over the past three decades wereeither machined from ram-extruded bar stock, or were machined frommolded blocks. Thus, the term “preformed polyethylene”, such as“preformed UHMWPE”, is used herein to mean UHMWPE which are used as thestarting materials from which implants are machined. Such preformedUHMWPE are consolidated by ram extrusion (also known as “ram extrudedUHMWPE”); and compression molding which is also known as block moldingof UHMWPE powder into bar, rods, sheets, blocks, slabs or the like(“block molded UHMWPE”). Thus, non-limiting examples of such preformedUHMWPE are: extruded UHMWPE bar, compression molded UHMWPE block andsheets.

The term “reference preformed UHMWPE” means conventional UHMWPE extrudedbars and conventional UHMWPE compression molded block.

The term “oxidation-resistant” as applied to polyethylenes (whichinclude HMWPE and UHMWPE, as in “oxidation-resistant polyethylene”,“oxidation-resistant HMWPE”, and “oxidation-resistant UHMWPE”) means apolyethylene that is more resistant to oxidation, for example, as causedby the free radicals that are induced during irradiation with a gammasource or an electron beam, than reference preformed UHMWPE. The implantmade from oxidation-resistant UHMWPE is also referred to as an“oxidation-resistant implant”.

Applicants' review of the current art show that the developers ofoxidation-resistant polyethylenes (discussed in “Background of theInvention”, above) have advocated their use in artificial jointsprimarily on the grounds that their resistance to oxidation results inless degradation of their mechanical properties and/or wear resistancewhen they are gamma irradiation sterilized in the range of 2.5 to 4Mrads that is standard in the industry. However, applicants realize thatthis method of manufacture does not improve the wear resistance overthat of typical gamma-sterilized implants of the past, which isunacceptably high in many patients, particularly the younger, moreactive patients who subject their joints to a high number of wear cyclesper year. Some advocates of oxidation-resistant UHMWPE have criticizedthe use of elevated levels of radiation crosslinking to improve wearresistance. For example, Mori et al stated that: “Recently the practiceof crosslinking UHMWPE after gamma-ray or electron-beam irradiation hasdeveloped to improve resistance to oxidation. However, the process ofcrosslinking of UHMWPE is complex, and it is difficult to remove thefree radicals created by irradiation. It is suggested that the additionof harmless Vitamin E is a very simple and extremely effective method toprevent oxidation degradation and maintain the stability of UHMWPEcomponent in artificial joints.” (Mori 1017, supra, paragraph bridgingleft and right columns. See also, Mori hand-out, second and third lastsentences).

On the other hand, crosslinking of a polyethylene is known to improvethe wear resistance in industrial implants. Applicants tested thefollowing: (1) chemically crosslinked acetabular cups produced by mixinga peroxide with UHMWPE powder and then molding the cups directly toshape; (2) gamma radiation-crosslinked acetabular cups produced byexposing conventional extruded UHMWPE bar stock to gamma radiation atvarious doses from 3.3 to 100 Mrad, remelting the bars to extinguishresidual free radicals (i.e., to minimize long-term oxidation), and thenmachining the cups by conventional techniques. In hip-joint simulatortests lasting as long as 5 million cycles, both types of crosslinkedcups exhibited dramatically improved resistance to wear. McKellop, H.,et al., “Development of An Extremely Wear-Resistant Ultra High MolecularWeight Polyethylene for Total Hip Replacements, J. Ortho. Res. Soc. 17:157-167(1999). For a review of conventional and industrial methods bywhich gamma or electron beam are used to irradiate conventional extrudedUHMWPE bar stock to increase its crosslinking and thus wear resistance,and then remelt or anneal the bar stock to extinguish residual freeradicals (i.e., to minimize long-term oxidation), and subsequentlymachining implants from the bar stock, and various studies using implantsimulator, see, e.g., H. A. McKellop, “Bearing Surfaces in Total HipReplacements: State of the Art and Future Developments”, AAOSInstructional Course Lectures, 50: 165-179 (2001); and Kurtz et al.,supra; and McKellop, H., et al., “Development of An ExtremelyWear-Resistant Ultra High Molecular Weight Polyethylene for Total HipReplacements, J. Ortho. Res. Soc. 17: 157-167(1999).

In the present invention, applicants recognize that, if one begins withan oxidation-resistant polyethylene and increases its wear resistance byincreasing its level of crosslinking above that generated by the dose ofradiation typically used to conventionally sterilize an implant (thatis, above the range of 2.5 to 4 Mrads), then it is possible to producean implant that has the desired high resistance to oxidation incombination with an improved wear resistance, making the implantparticularly suitable for use in younger and more active patients thanhas been advisable in the past. Thus the present invention provides fora wear-resistant and oxidation-resistant implant by fabricating theimplant from an oxidation-resistant polyethylene, including eithershaping (such as machining) it from a preformed polyethylene, or bydirect molding; packaging said implant, preferably in a low-oxygenatmosphere, and subjecting the packaged implant to a radiation doseabove the range used for conventional sterilization in order to increasethe level of crosslinking and, thereby, improve the wear resistanceabove that obtained with conventional radiation sterilization. Thus, thepresent invention provides for a way to produce oxidation-resistantimplants, orthopaedic materials comprising polyethylenes, andpolyethylenes that are more wear resistant than the conventionalpolyethylenes disclosed in the conventional arts, such as thosedescribed by the authors Mori et al., Tomita et al., and Walsh et al.,supra.

The method of the present invention is also easier and cheaper to carryout than the conventional methods:

In the present invention, since the polyethylene has been formed in amanner that renders it highly resistant to oxidation, despite thepresence of free radicals, there is no need to thermally treat thepolyethylene during or after radiation crosslinking (e.g., by annealingor remelting) in order to extinguish the residual free radicals, andthus simplifying the manufacturing process. In addition, in the presentinvention, it is possible to use direct molded polyethylene componentssuch as implants, which is not possible if remelting or annealing of theUHMWPE are required, since this may cause excessive distortion of animplant. In addition, since the oxidation-resistant polyethylene remainsoxidation-resistant despite the presence of free radicals, the presentinvention also has the advantage of allowing for the use of gamma orelectron beam to sterilize the component, rather than gas plasma orethylene oxide that are currently used to avoid the free radicalsgenerated by irradiation sterilization. The ability to use irradiationsterilization is an added advantage since gas plasma or ethylene oxidemay not be appropriate for some modular types of implants because of thelow penetration of gas plasma or ethylene oxide into the interfacesbetween the components of the implant.

Another advantage of the invention is that the irradiation crosslinkingof the finished implant to improve its wear resistance and theirradiation sterilization may be done simultaneously in a single step.Thus, the methods of the present invention are simpler and morecost-efficient than the conventional methods.

The following describes the invention in more detail:

III. Oxidation-Resistant Polyethylenes

A. Method A

Applicants note that there are publications claiming that direct moldingof UHMWPE produces a finished article of greater resistance to oxidationthan a finished article machined from an extruded bar or molded block.The improvement is due to a more uniform pressure and temperatureapplied to the finished article during direct molding.

This method is exemplified by—the commercially availableoxidation-resistant direct molded implant (commercially available fromZimmer, Inc., discussed in the “Background of the Invention”, above).

However, Mori p. 1122 has noted that direct molded (using 45 kgf/cm²pressure) specimens were less oxidation resistant than specimensmachined from conventional ram extruded bar stock. On the other hand,direct molded (using 200 kgf/cm² pressure) specimens were more oxidationresistant than specimens machined from conventional ram extruded barstock. Thus, Mori et al., reported that: “It is generally said thatdirect compression molded UHMWPE provide an unexpected resistance tooxidation. However, the results obtained from this study reveal thatgranted directly compression molding method was used, unsuitableconditions of the process could cause less resistance to oxidation.”(Mori p. 1122, supra, under the “Discussion” section).

In view of the concern expressed above in Mori p. 1122, for the presentinvention, Method A preferably has the added modification(s) of MethodB, below, in order to provide the suitable conditions for producingoxidation-resistant implants. Applicants also note that due to theconcern of Mori p. 1122, one skilled in the art should apply the routinetesting procedures described in the section “Testing and SelectionProcedures”, below, to ensure that he obtains his desiredoxidation-resistant and wear-resistant polyethylene and implant.

2. Method B

This Method B has four aspects to it, each of which represents amodification. In Method B, each of the aspects can be applied alone, orin combination with one or more of the other aspects, to polyethyleneprocessed by extrusion, by block molding, or by direct molding in orderto make an oxidation-resistant polyethylene or implant. Thus, Method Bcould be applied to Method A and/or Method C.

Method B: Aspect 1

Applicants realize that adding one or more antioxidants (for economicaland efficiency sake, one antioxidant is preferred), such as those knownin the chemical, polymer, and biological arts, would render thepolyethylene oxidation resistant. Non-limiting examples of conventionalantioxidants which may be used are vitamin A, vitamin C and vitamin E;and common antioxidants used to prevent or inhibit the oxidation ofpolymer, such as, members of the classes of phenols, aromatic amines,and salts and condensation products of amines and aminophenols withaldehydes, ketones, and thio compounds. Non-limiting examples of themembers of the foregoing classes are paraphenylene diamines anddiarylamines.

An example of the application of this aspect is found in Mori et al. andTomita et al. who used Vitamin E to improve the oxidation resistance oftheir UHMWPE. (Mori Hand-Out, Mori p. 1122, Mori p. 1017, and Tomita etal., J. Biomed Mater Res, supra.)

Method B: Aspect 2

This aspect seeks to reduce or remove the amount of oxygen present inthe polyethylene after it is fused (whether by extrusion, block molding,or direct molding, into, e.g., a solid bar, block, rod sheet or directmolded implant) in order to reduce the amount of oxidation that occursduring irradiation and thereafter. This can be done, for example, byplacing the powder in a suitable container and causing an inert gas toflow through the powder in order to flush out the air (which containsoxygen) that is mixed with the powder. In addition, whether or not thepowder has been flushed with inert gas, the container can be pressurizedwith an inert gas and maintained under pressure in order to force theinert gas into the powder, thereby displacing the air (which containsoxygen) from the powder. In addition, whether or not the powder has beenflushed and/or pressurized with inert gas, the container can beevacuated and maintained under vacuum in order to draw out oxygen thatis mixed with the powder and/or absorbed into the powder. In addition,whether or not one or more of the previous steps have been taken, thefusing of the powder (whether by extrusion, block molding or directmolding) can be done in a low oxygen atmosphere, such as an atmosphereof inert gas, to minimize mixing of air (which contains oxygen) with thepolyethylene during the fusing process. These procedures are notgenerally exclusive and are additive and will reduce oxygen present inthe polyethylene after it is fused such that the polyethylene implantwill be more resistant to oxidation during irradiation, and duringstorage prior to use, and during use in vivo. Alternatively, theresulting polyethylene or implant may be stored with an oxygen absorbentor in any one of conventional low-oxygen conditions, instead of storingin vacuum or inert gas.

An example of the application of this aspect is found in Sun et al.,U.S. Pat. No. 5,414,049, supra which discloses: “A medical implant madeof polymeric material having an increased oxidation resistance is formedby a method including the steps of placing a resin powder in a sealedcontainer. A substantial portion of the oxygen is removed from thesealed container by either a vacuum, an oxygen absorbent or by flushingwith inert gas. The container is then repressurized with a gas such asnitrogen, argon, helium or neon so that long term storage may bepossible. On use, the resin is transferred to a forming device whichboth melts and forms the resin in an oxygen reduced atmosphere toproduce a polymeric raw material such as a rod or bar stock. The medicalimplant is then formed from this raw material annealed and sealed in anairtight package in an oxygen reduced atmosphere.” (In the “Abstract”section of Sun et al., U.S. Pat. No. 5,414,049).

Method B: Aspect 3

This aspect fuses the polyethylene powder at a higher pressure, a highertemperature, and/or for a longer period of time than is conventionallyused in order to achieve better fusion of the polyethylene powder, sinceit has been shown that diffusion of oxygen into the polyethylene occurspreferentially along the boundaries between the poorly or incompletelyfused polyethylene powder or flakes. See, e.g., Mori p. 1122, supra. Inthis aspect of Method B, the pressure is preferably about 10 MPa orhigher, and more preferably about 14 MPa or higher; the temperature ispreferably about 175° C. or higher, and more preferably about 200° C. orhigher; the time is preferably about 30 minutes or longer, and morepreferably about 1 hour or longer.

An example of the application of this aspect is found in Mori p. 1122,supra, which showed that specimens custom molded directly from GUR1150resin under elevated compression pressure of 200 kgf/cm² had improvedoxidation index compared to those produced under compression pressure of45 kgf/cm². Applicants note that 45 kgf/cm² (about 4.4 MPa) is at thelow end of the range for conventional molding pressure, and moreimportantly, 200 kgf/cm² (about 19.6 MPa) is higher than theconventional molding pressure.

The use of elevated temperature, elevated pressure, and a longer periodfor fusing the polyethylene is also found in England et al.'s U.S. Pat.Nos. 5,466,530 and 5,505,984, discussed in further detail in Method C,below.

Method B: Aspect 4

When a polyethylene is irradiated, the irradiation generates freeradicals. The free radicals that are present in the amorphous regionsimmediately crosslink or oxidize. Thus, if the irradiation is conductedin low oxygen atmosphere, the free radicals in the amorphous regionswould tend to immediately crosslink instead of oxidize due to the lackof oxygen. The free radicals that have not crosslinked or oxidized afterirradiation are primarily trapped in the crystalline regions of thepolyethylene. Over time, these residual free radicals migrate to theamorphous regions. If the implant is removed from a low oxygenatmosphere after irradiation, these residual radicals can encounteroxygen molecules that have diffused into the amorphous regions, andoxidation can occur. Therefore, if there is a lower percentage ofcrystalline polyethylene, then there will be proportionately fewer freeradicals and, therefore, less long-term oxidation.

Thus, the present aspect of Method B seeks to reduce the percentage ofcrystalline polyethylene which in turn reduces the residual freeradicals in the polyethylene after irradiation, and which in turnreduces long-term oxidation. A non-limiting way to reduce thecrystallinity of a polyethylene is to quench (i.e., rapidly cool) apolyethylene after it has been heated and fused, which results in alower crystallinity in the quenched polyethylene.

An example of the application of this aspect is found in Example 1,below, wherein the molten UHMWPE specimen that had been molded at 170°C. and a ram pressure of 7.5 MPa for two hours was removed from thepress and very rapidly cooled (“quenched”) in a liquid nitrogen pool.

Another example of the application of this aspect is found in Walsh p.543, supra, which reported its directly molded UHMWPE samples were moreoxidation resistant than samples machined from extruded bar orcompression molded sheet. The direct molding of the UHMWPE samples wasconducted at 8.1 MPa, 165° C. and a cooling rate of greater than 175° C.per minute (“quenching”). Walsh p. 542, supra, also reported thatspecimens directly molded from 4150HP UHMWPE powder to provide a modulusof 600-800 MPa, was oxidation resistant.

Burstein and Li, U.S. Pat. No. 5,721,334, “Process for ProducingUltra-High Molecular Weight Low Modulus Polyethylene Shaped Articles ViaControlled Pressure and Temperature and Compositions and ArticlesProduced Therefrom”, was issued Feb. 24, 1998. The patent's co-inventor(Stephen Li) also co-authored Walsh p. 542 and 543, supra. This patentnoted that the shaped UHMWPE articles made from its method are useful asthe bearing surface of total knee joints, prosthetic hip joint cups, andas other prosthetic shapes for replacement of other joints of the humanbody. Significantly, the patent does not disclose that its methodsproduce oxidation-resistant UHMWPE. However, the patent discloses afive-step direct molding method which contains a quenching step 5. Themethod is described as follows: “1. Filling a mold with powdered UHMWPE;2. Covering the mold—with a piston; 3. Raising the temperature of themold . . . while simultaneously applying a pressure . . . to the moldand its contents; 4. Maintaining the pressure and temperature . . . ; 5.Dropping the temperature of the molded part, either completely or inpart at a rate between about 4° and about 175° C./minute, either whilemaintaining the mold at the molding pressure, at pressure less than themolding pressure, or under no external applied pressure. Dropping thetemperature of the molded part may be accomplished either by cooling thepart directly by contact with a suitable cooling fluid such as water orair or by cooling the mold.” (U.S. Pat. No. 5,721,334, supra, “Summaryof the Invention”, second. para.). Applicants note that the quenchingrate in step 5 of the patent (of about 175° C./minute) overlaps that ofWalsh p. 543, supra (greater than 175° C. per minute); and the modulusof the patented UHMWPE shaped article (elastic modulus of about 500 Mpato about 800 Mpa) overlaps that of Walsh p. 542, supra (modulus of600-800 Mpa). In view of the foregoing, applicants believe that thepatent's method could be used to produce oxidation-resistant UHMWPEimplants. The details and preferred ranges for the parameters forcarrying out the method are found in the patent to enable one skilled inthe art to produce the UHMWPE implants. One skilled in the art couldapply the routine testing procedures described in the section “Testingand Selection Procedures”, below, to test the UHMWPE implants producedunder the patented method to determine whether they areoxidation-resistant.

3. Method C

Applicants also recognize that if polyethylene powder is molded into asmaller size than the conventional size of UHWMPE blocks, the smallmolded polyethylene will have improved oxidation resistance due to amore uniform pressure and temperature applied to the finished articleduring direct molding. The small molded polyethylene is preferably about4-inch in diameter or smaller, and more preferably from about 0.5-inchto about 4-inch in diameter.

An example of the application of Method C is found in England, et. al.'sU.S. Pat. No. 5,466,530, “Biocompatible Components Fabricated from ASubstantially Consolidated Stock of Material” (issued Nov. 14, 1995) andU.S. Pat. No. 5,505,984, “Method for Forming Biocompatible ComponentsUsing An Isostatic Press” (issued Apr. 9, 1996). Even though thepatentees do not recognize that their methods for producingwear-resistant UHMWPE biocompatible component also produceoxidation-resistant component, applicants believe that their methods canproduce oxidation-resistant components. Thus, the patents' method can beapplied to produce implants and the implants can be tested according tothe method disclosed in the section “Testing And Selection Procedures”,below, to select for the oxidation-resistant implants. Applicants'belief is based on the fact that:

(1) the patents' method meets the criteria of Method C, herein, in thattheir molded UHMWPE were in smaller sizes, e.g., of 1.5-inch diameter to4-inch diameter, than the conventional′size of UHWMPE molded blocks; and

(2) their method (during its hot isostatic step) uses the elevatedpressure, temperature and a longer period for fusing the UHMWPE than arefavored in applicants' Method B, Aspect 3, above.

The England et al.'s method includes enclosing UHMWPE powder in a firstcontainer and subjecting the first container to a cold isostaticpressure treatment which forms an incompletely consolidated stock fromthe powder. The incompletely consolidated stock is removed from thefirst container and is placed in a second container which is thenlocated within a hot isostatic press and is subjected to a hot isostaticpressure treatment. The hot isostatic press treatment forms therelatively completely consolidated stock from the incompletelyconsolidated stock. The relatively completely consolidated stock is thenmachined into an implant. With regard to the size of the consolidatedstock, the patent states that: “For example, smaller diameters of thecompletely consolidated stock (e.g., 1½ inches) typically require lesstime to become fully compacted, while larger diameters of completelyconsolidated stock, such as 4 inches, typically require more time tobecome fully cured.” (U.S. Pat. No. 5,466,530, under “DetailedDescription of the Preferred Embodiment”, paragraph 26). The hotisostatic press operates from about 365° F. to about 420° F. When thehot isostatic press reaches approximately 365° F., the pressure is alsoincreased over a 1 to 2 hour period to a pressure preferably betweenabout 7,500 to about 10,000 psi (i.e., between about 52 to about 69MPa). Further details of the method and the preferred ranges ofparameters are to be found in the patents.

IV. Irradiation Steps

The oxidation-resistant polyethylene implant may be irradiated usingconventional methods known in the art, but modified to apply theradiation dose disclosed in the present invention. Non-limiting examplesof conventional irradiation methods are found in the references cited inthis application. For a review of industrial and conventionalirradiation methods as applied to polyethylene implants, see also, H. A.McKellop, AAOS Instructional Course Lectures, 50: 165-179 (2001), supra;and Kurtz et al., supra.

In order to minimize oxidation of the surface layer of the implantduring and after irradiation, the implant is preferably packaged in alow-oxygen atmosphere, for example, under a vacuum; in an inert gas suchas nitrogen, helium, or argon; or with an oxygen scavenger. Knownmethods for such low-oxygen atmosphere packaging may be used, such asthose used by the companies as described in “Background of theInvention”, above.

The irradiation for crosslinking and irradiation for sterilization stepsmay be performed separately (whether one precedes the other). However,for efficiency sake, preferably the packaged implant is exposed to aradiation source which simultaneously sterilizes and crosslinks theimplant to improve its wear resistance. Conventional radiation may beused, non-limiting examples of which are gamma radiation and electronbeam radiation. The preferred radiation is by gamma radiation. Asdiscussed above, conventional irradiation methods may be used, butmodified to apply the radiation dose disclosed in the present invention.It has been reported that comparable amounts of crosslinking of apolyethylenes, induced by gamma radiation or electron beam radiation,produced comparably low wear rates (For a review of the reports, see,e.g., H. A. McKellop, “Bearing Surfaces in Total Hip Replacements: Stateof the Art and Future Developments”, AAOS Instructional Course Lectures,50: 165-179 (2001); and Kurtz et al., supra).

Since the wear resistance of a polyethylene is known to be improved withincreasing levels of radiation-induced crosslinking, the radiation dosemay be higher than the range of 2.5 to 4 Mrads that is typically usedsimply to sterilize a polyethylene component.

The irradiation dose is preferably from about 5 to about 100 Mrad. Inthe case of UHMWPE, the radiation dose is more preferably from about 5to about 25 Mrad, and most preferably from about 5 to about 10 Mrad.This most preferable range is based on achieving a reasonable balancebetween improved wear resistance and minimal degradation of otherimportant physical properties. These ranges may be used as startingranges for other polyethylenes, as discussed further in the section “V.Testing and Selection Procedures”, below.

On the other hand, if a user is primarily concerned with reducing wear,and other physical properties are of secondary concern, then a higherdose than the above stipulated most preferable range (e.g., 5 to 10Mrad) may be appropriate. If the user is concerned with the physicalproperties and balancing them with wear resistance, he may modify theradiation dose according to the further teaching below. The optimumradiation dose is preferably based on the dose received at the level ofthe bearing surface in the final product.

Instead of using the above range of radiation dose as a criterion, theappropriate amount of crosslinking may be determined based on the degreeof swelling, gel content, or molecular weight between crosslinks afterirradiation. This alternative is based on the applicant's findings in WO98/01085, “Crosslinking of Polyethylene for Low Wear Using Radiation andThermal Treatments”, of the Orthopaedic Hospital et al., published Jan.15, 1998, and McKellop, H., et al., J. Ortho. Res. Soc. 17:157-167(1999), supra, that acetabular cups made from UHMWPE fallingwithin a preferred range of these physical parameters have reduced ornon-detectable wear. The ranges of these physical parameters preferablyinclude one or more of the following: a degree of swelling of betweenabout 1.7 to about 3.6; molecular weight between crosslinks of betweenabout 400 to about 3,500 g/mol; and a gel content of between about 95%to about 99%. A preferred polyethylene or final product has one or more,and preferably all, of the above characteristics.

These parameters can also be used as starting points for applying themethod (but absent the annealing or remelting of WO 98/01085, supra)described in the second aspect of the invention in WO 98/01085 fordetermining the desired radiation dose to balance the improvement inwear resistance with other desired physical or chemical properties, suchas polyethylene strength or stiffness.

The above considerations are further discussed in the following section.

V. Testing and Selection Procedures

Applicants recognize that different users, such as manufacturers, ofpolyethylene implants may have different criteria regarding theacceptable combination of oxidation resistance, wear resistance, andother important mechanical properties (such as fatigue resistance), andthese criteria may differ for different types of implants for a givenuser or manufacturer. Nevertheless, from the discussion herein, oneskilled in the art applying the present invention will know how to useestablished techniques for measuring oxidation resistance, wearresistance and other mechanical properties to determine whether thesemeet his individual criteria.

That is, one skilled in the art may use the methods disclosed in thepresent invention to make a candidate polyethylene, or he may choose amember of the polyethylene class as a candidate, and apply the followingroutine tests to select for an improved oxidation resistance and wearresistance polyethylene and an implant made therefrom of the presentinvention which meet his desired characteristics.

The following presents a non-limiting example of the routine selectionsteps he may use. He may choose a member of the polyethylene class as acandidate (“unmodified polyethylene”) or, if he wishes to modify thepolyethylene (“modified polyethylene”), he may choose one or more of thesteps described above (in Methods A, B, and C, discussed above) forincreasing the oxidation resistance of the polyethylene. These stepsare, for the most part, non-exclusive and additive in their ability toimprove the oxidation resistance of polyethylene. The polyethylene(whether modified or unmodified, as he desires) may be routinely testedfor its oxidation-resistance according to methods known in the art. Forexample, by irradiating the polyethylene to the desired level ofcrosslinking and then subjecting it to shelf aging, as described inExample 1, below, or to accelerated aging by exposure to elevatedtemperature in air (or, if desired, pressurized oxygen) for an extendedperiod of time and then measuring the resultant level of oxidation.Suitable methods for accelerated aging are known in the art and aredescribed in, for example, Sanford, W. M. et al., “Accelerated oxidativeaging testing of UHMWPE”, Trans. 41st Ann. Mtg. Orthopaedic Res. Soc.,p. 119 (1995); Sun. D. C., et al., “A simple accelerated aging methodfor simulations of long-term oxidative effects in UHMWPE implants”, 42ndAnn. Mtg., Orthopaedic Res. Soc., p. 493 (1996); and Walsh p. 542,supra.

The resultant amount of oxidation in the polyethylene may be measured,for example, by cutting the polyethylene specimen into segments and thenmeasuring the level of oxidation at and below the surface usingFourier-transform infrared spectroscopy, as described, for example, byShen, F. W. et al., “Potential Errors In FTIR Measurement of Oxidationin Ultrahigh Molecular Weight Polyethylene Implants”, J. BiomedicalMaterials Research, J. Applied Biomaterials, 48 (93), 203-210 (1999) orby measuring the density as a function of depth into the sample, asdescribed by Walsh p. 542 and 543, supra. This is advisable in view ofthe observation in Mori 1122, supra, that some directly moldedpolyethylene may not be oxidation-resistant.

As discussed above, the wear resistance of UHMWPE increases withincreased crosslinking {see, e.g.; McKellop, H., et al., J. Ortho. Res.Soc. 17: 157-167(1999), supra}. As discussed above, for the presentinvention, the amount of crosslinking radiation for UHMWPE may be fromabout 5 to about 100 Mrads, preferably from about 5 to about 25 Mrads,and most preferably from about 5 to about 10 Mrads. Thus, one skilled inthe art may use these ranges as a starting point for his irradiationstep for UHMWPE. The quantitative relationship between the level ofcrosslinking and the wear resistance may be different for polyethylenesother than UHMWPE, and can be determined by exposing the candidatepolyethylene to a range of radiation doses, preferably between about 5to about 100 Mrads, and performing suitable wear tests on each todevelop a dose-wear relationship.

Conventional wear tests such as sand slurry, pin-on-disk, and implantsimulator tests may be used to test the wear resistance of thepolyethylene specimens. Preferably, implants are directly molded fromthe polyethylene or machined from it, and the wear resistance as afunction of level of crosslinking may be determined by subjecting theimplants to laboratory wear simulation in a suitable simulator for theparticular implant to be tested. For example, the many knownconventional simulators may be used. For example, the bearing surface ofan implant for a hip joint, such as the medical acetabular cup implantmay be tested in the hip simulators described in McKellop, H., et al.,J. Ortho. Res. Soc. 17: 157-167 (1999), supra. Similarly, a kneesimulator is described in Desjardins, J. B., et al., “The Use Of AForce-Control Dynamic Knee Simulator To Quantify The MechanicalPerformance Of A Total Knee Replacement Design, During FunctionalActivity” J. Biomechanic 33(10): 1231-1242 (2000). For a review of wearand oxidation research involving simulators, see e.g., Kurtz et al.,supra.

In addition, since crosslinking may reduce other mechanical properties,such as fracture toughness, dose-response relationships may be developedfor these properties as well by subjecting specimens that have beencrosslinked to various levels to appropriate established tests of thespecific mechanical properties, which may be found, for example, in thepublished standards of the American Society for Testing and Materials(ASTM).

As noted above, the criteria for an optimum balance among oxidationresistance, wear resistance and other mechanical properties may varyamong different users and manufacturers, and among types of implants,such as hip joints compared to knee joints, for a given manufacturer oruser. Nevertheless, the optimum values of the preparation parameters forfabricating a polyethylene with adequate amounts of oxidationresistance, wear resistance and other mechanical properties may beroutinely determined as described above, and the number of specimens andtests required may be minimized by the application of establisheddesign-of-experiments (DOE) statistical techniques.

Having described the invention, the following examples are presented toillustrate and support the invention, and are not to be construed aslimiting the scope of the invention.

Example 1 Oxidation of Gamma-Irradiated Ultra-High Molecular WeightPolyethylene

Method and Materials

Original powder of medical-grade UHMWPE, GUR 415 (Hoechst, Tex., USA),with a weight-average molecular weight of approximately 6×10⁶, was usedas received. Specimens were compression molded according to thefollowing procedures. The original powder of UHMWPE was placed in a 1 mmthick circular mold (5 centimeter inside diameter) between two stainlesssteel plates that were covered with aluminum foil. The press waspre-heated to 170° C. and then the UHMWPE was compression molded for twohours at 170° C. and a ram pressure of 7.5 MPa applied on the mold andpolyethylene. After two hours, the pressure was increased to 15 MPa andthe specimen was slowly cooled in the press to room temperature.

Alternatively, for the quench-crystallized UHMWPE, the molten specimenthat had been molded at 170° C. and a ram pressure of 7.5 MPa for twohours was removed from the press and very rapidly cooled (“quenched”) ina liquid nitrogen pool. The specimens were gamma sterilized in air atroom temperature to an average dose of about 3.4 Mrad.

The properties of UHMWPE were analyzed before and immediately aftergamma sterilization. One set of gamma-sterilized specimens was stored onthe shelf in ambient air for about 6.5 years.

Thermal properties of UHMWPE with or without gamma sterilization wereanalyzed using differential scanning calorimetry. Samples weighing about6-9 mg were heated from 50° C. at 10° C./min in a differential scanningcalorimeter (Perkin-Elmer DSC-4) to 170° C. The melting temperature wasidentified from the peak of the melting endotherm. Indium was used forcalibration of the temperature and heat of fusion. The degree ofcrystallinity was calculated as the heat of fusion of UHMWPE sampledivided by that of an ideal polyethylene crystal (292 J/g).

The degree of crosslinking of the irradiated UHMWPE was assessed fromthe swell ratios of the materials. A sheet, approximately 1 mm thick andweighing about 0.4 grams, was cut out of the irradiated specimens, andthe sol-fraction was extracted by boiling in p-xylene for 72 hours, with0.5 weight percent of antioxidant (2,6-di-t-butyl-4-methyl phenol) addedto prevent oxidation. After extraction, the gel was transferred to freshp-xylene and allowed to equilibrate at 120° C. for 2 hours. The swollengel was then quickly transferred to a weighing bottle, covered, andweighed. The data was obtained as the average of five measurements.Samples were then soaked in acetone and dried at 60° C. in a vacuum ovento constant weight. The swell ratio was calculated as weight of swollengel divided by that of dried extracted gel, and the gel content wasdetermined from the ratio of the weight of the dried extracted to theinitial nonextracted sample. The swell ratio was used to calculate thenetwork chain density, number-average molecular weight betweencrosslinks and crosslink density, according to the theory of Flory andRehner {Shen et al., J. Polym. Sci., Polym. Phys., 34:1063-1077 (1996)}.

The oxidation of the shelf-aged gamma sterilized UHMWPE was assessed byFourier transform infrared spectroscopy (FTIR). Segments of theshelf-aged UHMWPE were embedded in poly(methyl methacrylate) (PMMA) andmicrotomed into 200-micron thick slices. Oxidation measurements weretaken using a transmission FTIR (Mattson Genesis II, Wisconsin, USA)mounted with a microscope (Spectra-Tech IR Plan, Spectra-Tech, Inc.,Connecticut, USA). Measurements were taken using a 0.1×0.1 millimeterwindow, starting at the surface and moving across the thickness in100-micron steps, using 64 scans summation at a resolution of 16 cm⁻¹.Oxidation was calculated as the height of the carbonyl absorption at1717 cm⁻¹ divided by the height of the reference peak at 2022 cm⁻¹.

Result and Discussion

The thermal properties, swell ratio and gel content are shown in Table1.

TABLE 1 Peak M.W. Gel Melting Between Con- Point Cystallinity SwellCrosslinks tent (° C.) (%) Ratio (g/mol) (%) Type of Without WithWithout With With With With UHMWPE Irrad. Irrad. Irrad. Irrad. Irrad.Irrad. Irrad. Slow - 132.6 135 49.2 55.8 8.4 21,753 70.8 cooled Quenched129.3 130 44.2 49.8 6.0 10,952 76.7 “Irrad.” Denotes irradiation “M.W.”denotes number-average molecular weight

The oxidation profiles of the shelf-aged gamma sterilized UHMWPE areshown in FIG. 1. As shown in FIG. 1, the quenched UHMWPE exhibited animproved oxidation resistance, with a reduction of about 57% in the peakoxidation, as compared to the slow-cooled UHMWPE.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity andunderstanding, it will be obvious that various modifications and changeswhich are within the skill of those skilled in the art are considered tofall within the scope of the appended claims. Future technologicaladvancements which allow for obvious changes in the basic inventionherein are also within the claims.

The invention claimed is:
 1. A method to improve the wear resistance of an implant made of a polyethylene, said method comprising the steps of: a) providing an oxidation-resistant implant; b) irradiating the oxidation-resistant implant at a radiation dose above standard sterilization dose but below about 100 Mrad in order to crosslink the implant to improve its wear resistance, wherein said oxidation-resistant implant is more resistant to oxidation than an implant machined from a reference preformed UHMWPE. 